1. Field of the Invention
The present invention relates generally to Nixe2x80x94Coxe2x80x94Cr base alloys and, more particularly, to a high strength, sulfidation resistant Nixe2x80x94Coxe2x80x94Cr alloy for long-life service at 538xc2x0 C. to 816xc2x0 C. The alloy of the present invention provides a combination of strength, ductility, stability, toughness and oxidation/sulfidation resistance so as to render the alloy range uniquely suitable for engineering applications where sulfur-containing atmospheres are life limiting.
2. Discussion of the Related Art
Over the years, researchers have continually developed alloys meeting requirements for both high strength at intermediate temperatures and corrosion resistance under severe environmental conditions. This quest for increasing performance is far from over as designers and engineers continuously seek to increase productivity, lower operating costs, improve yields and extend service lives. All too often, however, researchers terminated their efforts when the target combination of properties was achieved. Such is the case, for example, in two industrial areas in critical need of advanced alloys to maintain progress. These industrial applications are diesel exhaust valves and alloys for coal-fired boilers. These applications have in common that their developers require ever-increasing strength at increasingly higher temperatures, improved resistance to sulfur-containing atmospheres as atmospheres become more demanding and increases in service lives to assure trouble-free operation over the life of the equipment. Heavy-duty diesel engines in off-road construction equipment, often operating in remote corners of the globe where refined, low sulfur fuels are not available, are suffering exhaust valve failure due to sulfidation attack. Maintenance of these engines, usually requiring original equipment mechanics, can become prohibitively expensive and time-consuming. These same engines are now being designed for higher temperatures to increase power and efficiency. This has only served to exacerbate the alloy challenge.
Ultra supercritical boiler designers are creating a similar problem in coal-fired boilers as utilities seek to improve efficiency by raising steam pressure and temperature. Today""s boilers with efficiencies around 45% typically operate at a 290 bar steam pressure and 580xc2x0 C. steam temperature. Boiler designers are setting their sights on 50% efficiency or better by raising the steam conditions as high as 375 bar/700xc2x0 C. To meet this requirement in the boiler tubing, the 100,000 hour stress rupture life must exceed 100 MPa at 750xc2x0 C. (mid radius tube wall temperature needed to maintain a 700xc2x0 C. steam temperature at the inner wall surface). Raising steam temperature has made coal ash corrosion more troublesome, placing a further requirement on any new alloy. This corrosion requirement is less than 2 mm of metal loss in 200,000 hours for exposures in the temperature range of 700xc2x0 C. to 800xc2x0 C. For economy, the boiler tube must be as thin-walled as possible (i.e.,  less than 8 mm wall thickness) and be fabricable into long lengths in high yield on conventional tube making equipment. This places a major constraint on the maximum work-hardening rate and yield strength tolerable in manufacture and field installation, physical property characteristics running counter to the need for superior strength in valve and boiler tube service.
To meet the new strength and temperature requirements of an advanced diesel exhaust valve or a future boiler tube alloy, designers must exclude the usual ferritic, solid solution austenitic and age-hardenable alloys heretofore employed for this service. These materials commonly lack one or more of the requirements of adequate strength, temperature capability and stability or sulfidation resistance. For example, the typical age-hardenable alloy, in order to develop high strength at intermediate temperatures, must be alloyed with insufficient chromium for peak sulfidation resistance in order to maximize the age-hardening potential of the alloy. Adding chromium not only degrades the strengthening mechanism but, if added in excess, can result in embrittling sigma, mu or alpha-chromium formation. Since 538xc2x0 C. to 816xc2x0 C. is a very active range for carbide precipitation and embrittling grain boundary film formation, alloy stability is compromised in many alloys in the interest of achieving high temperature strength and adequate sulfidation resistance.
The present invention overcomes the problems of the prior art by providing a Nixe2x80x94Coxe2x80x94Cr-base alloy range possessing exceptional resistance to sulfur-containing atmospheres containing limiting amounts of Al, Ti, Nb, Mo and C for high strength at 538xc2x0 C. to 816xc2x0 C. while retaining ductility, stability and toughness.
The present invention contemplates a newly-discovered alloy range that extends service conditions for the above-described critical industrial applications notwithstanding the seemingly incongruous constraints imposed by the alloying elements economically available to the alloy developer. Past alloy developers commonly claimed broad ranges of their alloying elements which, when combined in all purported proportions, would have faced these counter influences on overall properties. The present inventors have discovered that a narrow range of composition does exist that allows one to fabricate a high strength alloy for service at 538xc2x0 C. to 816xc2x0 C. with both sulfidation resistance, phase stability and workability. A better appreciation of the alloying difficulties can be presented by defining below the benefits and impediments associated with each element employed in the present invention.
A high strength, sulfidation resistant Crxe2x80x94Coxe2x80x94Ni base alloy for long-life service at 538xc2x0 C. to 816xc2x0 C. containing, in % by weight, about 23.5-25.5%Cr, 15.0-22.0%Co, 0.2-2.0%Al, 0.5-2.5%Ti, 0.5-2.5%Nb, up to 2.0%Mo, up to 1.0%Mn, 0.3-1.0%Si, up to 3.0%Fe, up to 0.3%Ta, up to 0.3%W, 0.005-0.08%C. 0.01-0.3%Zr, 0.001-0.01%B, up to 0.05% rare earth as misch metal, 0.005-0.025%Mg plus optional Ca, balance Ni, including trace additions, such as up to 0.05%La, up to 0.05%Y, plus impurities. The alloy provides a combination of strength, ductility, stability, toughness and oxidation/sulfidation resistance so as to render the alloy range uniquely suitable for engineering applications where sulfur-containing atmospheres are life limiting.
The combination of elements set forth above unexpectedly and surprisingly possesses all of the critical attributes required of high strength applications in sulfur-containing atmospheres. It has been discovered that sulfidation resistance can be achieved by alloying within a narrow range of Cr (23.5-25.5%Cr) without destroying phase stability resulting from embrittling phases by concurrently limiting certain elements to very narrow ranges, namely, Mo to less than 2%, C to less than 0.08%, Fe to less than 3.0% and the total Ta plus W content to less than 0.6%. Less than 23.5%Cr results in inadequate sulfidation resistance and greater than 25.5%Cr produces embrittling phases even with the alloy restrictions defined above. It should be mentioned that, unless otherwise specified, all percentages of the various alloy constituents set forth herein are percent by weight.
Oftentimes, in striving for maximum corrosion resistance, the resultant alloys lack the required high temperature strength. This has been solved by the instant invention by balancing the weight percent of precipitation hardening elements to a narrow range where the resulting volume percent of hardening phase is between about 10 and 20% within the Nixe2x80x94Coxe2x80x94Cr matrix. Excessive amounts of the hardener elements not only reduce phase stability and lower ductility and toughness, but also render valve and tubing manufacturability extremely difficult, if not impossible. The selection of each elemental alloying range can be rationalized in terms of the function each element is expected to perform within the compositional range of the present invention. This rationale is defined below.
Chromium (Cr) is an essential element in the alloy of the invention because Cr assures development of a protective scale which confers the high temperature oxidation and sulfidation resistance vital for the intended applications. In conjunction with the minor elements Zr (up to 0.3%), Mg (up to 0.025%) and Si (up to 1.0%), the protective nature of the scale is even more enhanced and made effective to higher temperatures. The function of these minor elements is to enhance scale adhesion, scale density and resistance of the scale to decomposition. The minimum level of Cr is chosen to assure xcex1-chromia scale formation at 538xc2x0 C. and above. This level of Cr was found to be about 23.5%. Slightly higher Cr levels accelerated xcex1-chromia formation but did not change the nature of the scale. The maximum Cr level for this alloy range was determined by alloy stability and workability. This maximum level of Cr was found to be about 25.5%.
Cobalt (Co) is an essential matrix-forming element because Co contributes to hot hardness and strength retention at the upper regions of the intended service temperature (538xc2x0 C.-816xc2x0 C.) and contributes in a significant way to the high temperature corrosion resistance of the alloy range. However, because of cost, it is preferred to maintain the level of Co below 40% of that of the Ni content. Thus the beneficial range of the Co content becomes 15.0-22.0%.
Aluminum (Al) is an essential element in the alloy of the present invention not only because Al contributes to deoxidation but because it reacts with nickel (Ni) in conjunction with Ti and Nb to form the high temperature phases, gamma prime (Ni3Al,Ti,Nb) and eta phase (Ni3Ti,Al,Nb). The Al content is restricted to the range of 0.2-2.0%. The minimum total of elements contributing the hardening elements are related by the following formula:
%Al+0.56xc3x97x%Ti+0.29xc3x97%Nb=1.7%, preferably xe2x89xa72.0%xe2x80x83xe2x80x83(1)
While the maximum hardening elements are related by the following formula:
%Al+0.56xc3x97%Ti+0.29xc3x97%Nb =3.8%, preferably  less than 3.5%xe2x80x83xe2x80x83(2)
Larger amounts than 2.0%Al in conjunction with the other hardener elements markedly reduce ductility, stability and toughness and reduce workability of the alloy range. Internal oxidation and sulfidation can increase with higher amounts of Al.
Titanium (Ti) in the range 0.5-2.5% is an essential strengthening element as defined in equations (1) and (2), above. Ti also serves to act as grain size stabilizer in conjunction with Nb by forming a small amount of primary carbide of the (Ti,Nb)C type. The amount of carbide is limited to less than 1.0 volume % in order to preserve hot and cold workability of the alloy. Ti in amounts in excess of 2.5% is prone in internal oxidation to leading to reduced matrix ductility.
Niobium (Nb) in the range 0.5-2.5% is also an essential strengthening and grain size control element in the alloy of the present invention. The Nb content must fit within the constraints of equations (1) and (2), above, when Al and Ti are present. Nb along with Ti can react with C to form primary carbides which act as grain size stabilizers during hot working. Compositions 2 through 4 of Table IIB contain increasing amounts of Nb which, when one examines the flue gas/coal ash corrosion data of Table VI, finds that Nb has a negligible effect on the rate of corrosion within the limits of the present invention. Table VI presents metal loss and depth of attack for 2,000 hours at 700xc2x0 C. in a flue gas environment of 15%CO2B4%O2B 1.0%SO2Bbal.N2 flowing at the rate of 250 cubic centimeters per minute. The specimens were coated with a synthetic ash comprising 2.5%Na2SO4+2.5%K2SO4+31.67%Fe2O3+31.67%SiO231.67%Al2O3. An excessive amount of Nb can reduce the protective nature of protective scale and, hence, is to be avoided. Tantalum and W also form primary carbides which can function similarly to that of Nb and Ti. However, their negative effect on xcex1-chromia stability limits their presence of each to less than 0.3%.
Molybdenum (Mo) can contribute to solid solution strengthening of the matrix but must be restricted to less than 2.0% due to its apparent deleterious effect on oxidation and sulfidation resistance when added in greater amounts to the alloys of the present invention. Table V shows the reduction in sulfidation resistance as a function of Mo content based on metal loss and depth of attack after times to 3,988 hours at 700xc2x0 C. in a flue gas environment of 15%CO2B4%O2B 1.0%SO2Bbal.N2 flowing at the rate of 250 cubic centimeters per minute. The specimens were coated with a synthetic ash comprising 2.5%Na2SO4 +2.5%K2SO4+31.67%Fe2O3+31.67%SiO2+31.67%Al2O3.
Manganese (Mn), while an effective desulfurizer during melting, is overall a detrimental element in that it reduces protective scale integrity. Consequently, the Mn content is maintained below 0.5%. Mn, above this level, degrades the xcex1-chromia phase by diffusing into the scale and forming a spinel, MnCr2O4. This oxide is significantly less protective of the matrix than is xcex1-chromia.
Silicon (Si) is an essential element in the alloy according to the present invention because Si ultimately forms an enhancing silica (SiO2) layer beneath the xcex1-chromia scale to further improve corrosion resistance in oxidizing and sulfidizing environments. This is achieved by the blocking action that the silica layer contributes to inhibiting ingress of the molecules or ions of the atmosphere and the egress of cations of the alloy. Levels of Si between 0.3 and 1.0% are effective in this role. Excessive amounts of Si can contribute to loss of ductility, toughness and workability.
Iron (Fe) additions to the alloys of the invention lower the high temperature corrosion resistance by reducing the integrity of the xcex1-chromia scale by forming the spinel, FeCr2O4. Consequently, it is preferred that the level of Fe be maintained at less than 3.0%.
Zirconium (Zr) in amounts between 0.01-0.3% and boron (B) in amounts between 0.001-0.01% are effective in contributing to high temperature strength and Stress rupture ductility. Larger amounts of these elements lead to grain boundary liquation and markedly reduced hot workability. Zr in the above compositional range also aids scale adhesion under thermally cyclic conditions. Magnesium (Mg) and optionally calcium (Ca) in a total amount between 0.005 and 0.025% are both an effective desulfurizer of the alloy and a contributor to scale adhesion. Excessive amounts of these elements reduce hot workability and lower product yield. Trace amounts of La, Y, or misch metal may be present in the alloys of the invention as impurities or as deliberate additions up to 0.05% to promote hot workability and scale adhesion. However, their presence is not mandatory as is that of Mg and optionally Ca.
Carbon (C) should be maintained between 0.005-0.08% to aid grain size control in conjunction with Ti and Nb since the carbides of these elements are stable in the hot working range (1000-1175xc2x0 C.) of the alloys of the present invention. These carbides also contribute to strengthening the grain boundaries to enhance stress rupture properties.
Nickel (Ni) forms the critical matrix and must be present in an amount greater than 45% in order to assure phase stability, adequate high temperature strength, ductility, toughness and good workability.